I did a google image search using "evil cupcake" and it came back with a tattoo pic of a frosted cupcake that had horns, mini-skulls sprinkled on the frosting, and a tail coming out the back (you could probably make one wrap around the top of the cupcake instead). Just an idea...
Edited: I really need to learn how to spell. 
black icing, horns and tail coming out (as if the devil is inside the cupcake)
Leah16: do be careful with the decorations you choose, specifically the colours (using too much colour paste can make your icing bitter). also, be sure to think through how you're going to transport your little cakes so they don't get squished before you get a chance to pass them around the classroom.
i'm not sure which Satanism your presentation is about (the worship of the Christian Fallen Angel? The beliefs of the Church of Satan?). if it's the former, then yeah, go for pitchforks and horned demon imagery -- angry eyebrows always make a face look more menacing; but if it's the latter, then just making incredibly delicious chocolate cupcakes will accurately represent the sensuality aspect of Satanists' beliefs (which are based on a pre-Christian, Pagan notion of Satan as a force of nature, and are not really related to the Christian idea of Satan).
whichever way you go, you're going the right way: i always made food for my project presentations in high school -- your teacher will remember it, and your classmates will tend to be better behaved and more attentive during your presentation when they've got a treat to eat!
have fun! 
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